1. Field of the Invention
The present invention relates generally to semiconductors and more specifically to a method for producing an insulating trench in an SOI substrate.
2. Description of the Related Art
For integrating components in SOI substrates having a monocrystalline silicon layer, an insulating layer of SiO.sub.2 arranged thereon and a monocrystalline silicon layer arranged on the insulating layer of SiO.sub.2, the integrated components are produced in the monocrystalline silicon layer. Neighboring components or component groups are completely electrically insulated from one another vertically by the insulating layer and laterally by trenches that completely surround the respective component or component group. The trenches usually extend down onto the insulating layer and are filled with SiO.sub.2. The term dielectric insulation has become standard for this insulation.
In Smart Power Technology, complex logic components are integrated monolithically in a substrate with high-voltage power components. Since the logic components are operated with voltage levels around five volts but voltages of up to 500 volts occur in the high-voltage power components, an electrical separation of the high-voltage components from the logic components is required. Given SOI substrates, dielectric insulation is used for electrical separation of components in Smart Power Technology (see, for example, A. Nakagawa et al. ISPS 1990, pages 97-101).
In SOI substrates produced according to the direct wafer bonding (DWB) method (see, for example, A. Nakagawa et al. ISPS 1990, pages 91-101), the layer thickness of the monocrystalline silicon layer of SOI substrates is typically 20 .mu.m. This means that 20 .mu.m deep trenches must be etched by dry-etching into the monocrystalline silicon layer for producing insulation trenches that surround components to be insulated.
In order to assure that the monocrystalline silicon layer is completely etched through everywhere on the SOI substrate, the etching duration in this process step is usually lengthened by 10-50 percent compared to the value at which the monocrystalline layer is just etched through. Dry-etching processes that attack selectively relative to SiO.sub.2 are used for trench etching. The trench etching therefore stops at the surface of the insulating layer of the SOI substrate side. The dry-etching processes used etch anisotropically. In addition to the principal vertical etching component, however, these etching methods also have a lateral part. In the over-etching, this leads to under-etchings of the monocrystalline silicon layer forming at the surface of the insulating layer. These under-etchings of the monocrystalline silicon layer lead to a widening of the cross section of the trench at the surface of the insulating layer.
During the further course of processing, these under-etchings lead to difficulties, particularly given a thermal oxidation of the sidewalls of the trench. In the thermal oxidation of the sidewalls of the trench, a so-called bird's beak is formed at the floor of the trench in the region of the under-etchings. Since a trench mask that contains a Si.sub.3 N.sub.4 layer is usually used for trench etching, which protects the surface of the monocrystalline silicon layer during the thermal oxidation of the sidewalls, the formation of a bird's beak also occurs at the upper edge of the trench during the thermal oxidation. The formation of the bird's beak is a consequence of the increase in volume during the oxidation. This bird's beak leads to mechanical stresses at the upper edge of the trench as well as at the floor of the trench. These stresses at the upper edge of the trench can be reduced by deformation of the relatively thin mask layer. This, however, is not possible at the floor of the trench.
As a consequence of mechanical stresses, crystal defects form in the monocrystalline silicon layer at the upper edge and at the floor of the trench. The disturbance of the crystal lattice is greater when the mechanical stresses are higher. These crystal defects deteriorate the function of the components integrated in the monocrystalline silicon layer; they reduce the yield.
EP 0459397 A2 discloses that the mechanical stresses due to the formation of a bird's beak at the upper edge of the trench can be avoided in the thermal oxidation of the surface of a trench that is etched into a substrate of monocrystalline silicon by a bevelling of the upper edge of the trench by using incipient etching.
Y. Tamaki et al, J. Electrochem. Soc. 135, page 726 (1988) discloses that the upper edge of the trench can be bevelled by incipient etching and the trench floor can be rounded. This is done to avoid mechanical stresses due to the arising SiO.sub.2 in the thermal oxidation of the surface of a trench that is etched into a substrate of monocrystalline silicon.
U.S. Pat. No. 5,061,653 discloses a field oxide bird's beak fashioned laterally of a trench that is etched into a substrate of monocrystalline silicon. The trench has an oxidized surface. As a result thereof, mechanical stresses that occur due to the oxidation of the surface of the trench are reduced at the surface of the substrate.
Additional steps that increase the complexity of the process are introduced in all these methods. The appearance of the under-etchings at the trench floor when trenches are etched into the monocrystalline silicon layer of an SOI substrate is related to the insulating layer of SiO.sub.2 arranged under the monocrystalline silicon layer. The under-etchings are related to the necessary step of etching completely through the monocrystalline layer for the dielectric insulation. Such under-etchings do not occur when etching trenches in substrates of monocrystalline silicon. The measures known from the above three references are not suitable for dismantling mechanical stresses in the region of the under-etchings given a trench in an SOI substrate.